Genetic or pharmacologic inhibition of ceramide-mediated cell death within the vascular endothelium of the gastrointestinal tract (GI) protects mice from death from the Radiation GI Syndrome (RGS). Prophylactic administration of anti-ceramide antibody attenuates endothelial apoptosis, resulting in significant protection of intestinal stem cells and animal survival following lethal radiation exposure. Administration 24 h post radiation exposure similarly protects crypt stem cells and dramatically improves survival, indicating that anti-ceramide antibody represents the first effective antibody of small molecule mitigator of lethal RGS. To improve upon the efficacy of anti-ceramide antibody, we propose to select a recombinant single-chain antibody fusion protein containing the known ceramide-binding regions of anti-ceramide antibody as a novel Medical Radiation Countermeasure. Single-chain antibody fusion proteins are smaller derivatives of full length antibodies, and thus offer the advantage of rapid entering into the bloodstream and increased penetration into tissue compared to full-length antibodies. Additionally, single-chain antibody fusion proteins can be modified by general cloning techniques to display preferred target binding, and from a product development standpoint these fusion proteins can be produced easily and at minimal cost. While in some cases single-chain antibody fusion proteins may not retain full activity of the parent antibody, our preliminary data indicates that similar antibody fragments derived by enzymatic digestion of humanized anti-ceramide antibody results in significant protection and mitigation of radiation-induced crypt stem cell lethality, even 24 h after high single-dose exposure. These data indicate that a single-chain antibody fusion protein can be sufficient to induce therapeutic neutralization of ceramide. As such, a neutralizing anti-ceramide single-chain antibody fusion protein represents a promising candidate to fulfill the Project BioShield mandate for development of countermeasures to mitigate acute radiation syndromes within the first 24 h after a nuclear disaster.